A powder conveying device, which is also called pump equipment for powder, is known from the state of the art DE 103 00 280 A1. The pump equipment described there comprises two mechanically synchronized membrane pumps, which alternately convey the coating powder. Each of the two membrane pumps has an inlet valve, a dosing chamber and an outlet valve. If the inlet valve is opened and a negative pressure is generated in the dosing chamber with the help of a disk shaped membrane, the powder is sucked into the dosing chamber. Subsequently, the inlet valve is closed and with the help of compressed air, which is blown into the dosing chamber, the coating powder from the dosing chamber is blown out from the dosing chamber via the outlet valve that is now opened. The movement of the membranes of the two membrane pumps takes place over a piston rod, which connects the two membranes together. Due to this, if the chamber volume is increased in the first dosing chamber with the help of the membrane, the chamber volume in the second dosing chamber reduces automatically. This type of conveying device for powders is complex to manufacture. Moreover, a considerable time and effort is necessary for controlling the drive, that is to say the piston rod. Four sensors are necessary for detecting the position of the piston rod alone. Another disadvantage is that the conveying device must be cleaned in case of a color change and that is time consuming.
A conveying device for coating powders with a hose membrane pump is known from the post-published state of the art DE 103 53 968 A1, which has a powder inlet valve at the powder inlet and a powder outlet valve at the powder outlet of a conveying chamber. Both the powder inlet valve as well as the powder outlet valve are developed as passive valves, which are opened by the flow pressure of the conveyed powder and close themselves automatically again, if the flow pressure of the powder is sufficiently low.
An alternative device for conveying powder is described in the patent application WO 03/024612 A1. It concerns a so-called piston pump, which has a first piston led into a first conveying chamber and a second piston led into a second conveying chamber. The powder is conveyed here alternately with the help of negative pressure generated through the first and/or through the second piston during their suction stroke. Subsequently, the powder is blown out, with the help of compressed air, from the conveying chamber in the direction of the powder spray gun. After that the one piston goes back into its initial position while the other piston executes a suction stroke. A disadvantage in this embodiment is that a seal is required between the conveying chamber and the conveying piston in order to prevent the powder between conveying chamber and conveying piston from escaping from the conveying device. However, in this case the danger exists that the powder is to be deposited at the seal and there could be an undesirable hardening of the powder and a caking of the powder at the conveying piston, the seal or the walls of the conveying chamber. Furthermore, the described powder conveying device is time consuming to clean in case of a color change.
In addition, devices for conveying powder are also known, which work according to the Venturi principle. In this case, in one so-called injector, negative pressure is generated in the injector with the help of a continuous conveying air flow and that leads to powder being sucked in from a powder reservoir and being conveyed, together with the conveying air flow, in the direction of the powder spray gun. However, the conveying device working according to the Venturi principle has the disadvantage that the suctioning capacity is relatively low and that, moreover, the length of the hose between the powder conveyor and the powder spray gun may not become too long.